The storage of valuable liquids in naturally occurring or solution-mined subterranean cavities was very well known. Typically, the cavity held in separate phases the valuable liquid and an immiscible displacing liquid, such as saturated brine, to entirely fill the cavity. When it was desired to introduce additional valuable liquid into the cavity, a corresponding volume of brine was simultaneously withdrawn. Conversely, when it was desired to withdraw valuable liquid from the cavity, it was displaced therefrom with a corresponding volume of brine introduced simultaneously into the cavity.
Depending on the density of the valuable liquid relative to the displacing liquid, the valuable liquid was disposed either over or under the displacing liquid, as described in U.S. Pat. No. 3,491,540.
For example, U.S. Pat. No. 3,745,770 described the storage of ethylene dichloride under brine, and U.S. Pat. Nos. 2,986,007 and 2,787,455 described the storage over brine of liquified petroleum gas or another light fluid which gasified on release of pressure. However, because of permanent losses associated with the rubble pile and other discontinuities at the bottom of the cavity, it was generally desirable to store the valuable liquid above the brine or other displacing liquid.
Such storage systems and methods were generally acceptable (except for permanent losses associated with storage under brine as previously noted) when the differences between the densities of the brine and the valuable liquid at the storage conditions was substantial.
Because of the risk of phase inversion, however, such storage of liquids having a density close to that of the brine at the storage conditions was impractical. For example, a liquid such as ethylene dichloride is more dense than saturated brine at ambient conditions, and also at slightly elevated temperatures, such as those occurring in cavities relatively close to the earth surface, and it has been conveniently stored under brine in shallow, low temperature cavities. However, at higher temperatures which generally occur in deeper cavities, the ethylene dichloride may have a density less than or about the same as the saturated brine, resulting in phase inversion, or in dispersion of ethylene dichloride droplets into the brine phase or vice versa. Moreover, the temperature in such cavities is seldom uniform, generally being lower at the top than at the bottom of the cavity. Liquid near the bottom of the cavity would be heated and would rise, due its decreased density, into the cooler liquid in the upper portion until it cooled sufficiently to be more dense than the liquid in the upper portion, whereupon it would begin falling and descend into the lower portion to repeat the cycle.
One attempt to solve the problem of the risk of phase inversion has been the use of a displacing fluid other than brine. In cavities mined in salt domes or spires, fresh water could not be used because it would eventually saturate by dissolution of minerals from the roof and walls of the cavity. On the other hand, organic liquids were generally too expensive to be practically considered.
Another attempt has been the modification of the stored material to increase its density so that the stored material would be disposed beneath the brine. This approach has been successful in the case of solids, such as, for example, the weighting of asbestos fibers with particulate material as described in U.S. Pat. No. 3,887,462, and in the case of a heavy condensable gas, such as, for example, chlorine compressed to maintain it in liquid form by the hydrostatic pressure of the brine disposed thereabove as described in U.S. Pat. No. 3,151,462. Heretofore, no such modification of valuable liquids to be stored in subterranean cavities with brine has been known.